1. Field of the Invention
This invention relates to a power conversion system, and more particularly to a current source power conversion system.
This invention also relates to a control device for a power conversion system with a plurality of unit converters whose AC terminals are connected in common, and more particularly to a control device for a power conversion system which controls the total AC output current of the power conversion system to an instruction value.
2. Description of the Related Art
FIG. 3 is a layout diagram of the main circuitry of a power conversion system in which a prior art example and this invention are applied in common. In FIG. 3, 1 is an AC load and 2 to 4 are capacitors. 5 to 8 are unit converters (hereinbelow simply called converters) that convert DC power to AC power. Parallel operation of converters 5 to 8 is performed by connecting their respective AC terminals in common to AC load 1. Capacitors 2 to 4 are employed for absorbing the switching surges of converter 5 to converter 8. 9 to 32 are self-turn-off switching devices that constitute converter 5 to converter 8. Hereinbelow, the case will be described in which gate turn-off thyristors (hereinbelow simply called GTOs) are employed as self-turnoff switching devices. 33 to 40 are DC reactors for smoothing the DC current. 41 to 44 are DC power sources.
FIG. 13 is a layout diagram of a prior art control circuit for controlling the power conversion system shown in FIG. 3. In FIG. 13, 69 is a current instruction value generating circuit for converter 5 to convertor 8, 70 is a phase detecting circuit, 71 is a triangular wave generator, 72 is a comparator, and 73 is a logic circuit for generating output instructions of the AC currents of converter 5 to converter 8.
FIG. 14 is a waveform diagram of the case where the power conversion system shown in FIG. 3 is controlled by the prior art control circuit shown in FIG. 13.
Hereinbelow, the operation of this prior art control circuit will be described with reference to FIG. 3, FIG. 13 and FIGS. 14A-E.
In FIG. 13, current instruction value generating circuit 69 generates an amplitude instruction value S1 and a phase angle instruction value of the AC current. Amplitude instruction value S1 is supplied to comparator 72. Phase angle instruction value is supplied to phase detection circuit 70 and triangular wave generator 71. Signals S2 to S5 are The output signals of triangular wave generator 71 and are triangular waves whose period is 60.degree. of the phase angle instruction value. Triangular waves S3 to S5 are respectively lagging in phase by 15.degree. in each case with respect to triangular wave S2. Comparator 72 compares amplitude instruction value S1 with triangular waves S2 to S5, and provides an output instruction of AC current in the range where the amplitude instruction value is larger than the triangular wave. Output instruction from comparator 72 and output of phase detection circuit 70 are supplied to logic circuit 73, which generates AC current output instructions for each converter by performing phase discrimination by means of the output of phase detection circuit 70. Specifically, S6 is s U phase output instruction of converter 5, S7 is an X phase output instruction of converter 5, S8 is a U phase output instruction of converter 6, S9 is an X phase output instruction of converter 6, S10 is a U phase output instruction of converter 7, S11 is an X phase output instruction of converter 8, S12 is a U phase output instruction of converter 8, and S13 is an X phase output instruction of converter 8.
Current of the waveform shown at S14 of FIGS. 14A-E is obtained as U phase output current by on/off control of GTOs 9-32 of converter 5 to converter 8 in accordance with the above output instructions S6-S13. Identical control is performed in respect of the V phase and W Phase, with their respective phases being made to lag by 120.degree. in each case with respect to the U phase.
As described above, when operation is performed with the AC terminals of converter 5 to converter 8 connected in parallel and with the conduction phases of the GTOs respectively lagging by 15.degree. in each case, the waveform of the output current of the power conversion system i.e. the resultant value of the output currents of the converters is a square-wave waveform as shown at S14 of FIGS. 14A-E, containing low-order, fifth and seventh higher harmonics.
Next, another power conversion system is described.
FIG. 15 is a layout diagram of a prior art example of a power converter. In this Figure, 101 is a power source, 102 is a transformer, 103 is a converter that converts AC power to DC power, 104 is an inverter that converts DC power to AC power, 105 is a DC reactor that smooths the DC current that flows from converter 103 to inverter 104, 107 to 112 are thyristors constituting converter 103, 113 to 118 are thyristors constituting inverter 104, 119 is a control circuit of converter 103, 120 is a control circuit of inverter 104, and 164 is a synchronous motor. Converter 103 and inverter 104 constitute a so-called externally commutated current source power converter, in which DC current smoothed by a DC reactor 105 is commutated and converted to AC current depending on an AC voltage.
FIGS. 16A-G is a waveform diagram illustrating the action of the prior art example shown in FIG. 15. In this Figure, VSUV (FIG. 16(A)) is a voltage between U and V phases of power source 101. A VW between-phase voltage and a WU between-phase voltage have the same waveform as voltage VSUV, lagging in phase by 120.degree. and 240.degree., respectively. ISU (FIG. 16(B)) is a current flowing in the U phase of converter 103. The same currents ascurrent ISU also flow in the V phase and W phase of converter 103, but lagging in phase by 120.degree. and 240.degree. with respect to current ISU, respectively. VDC is a DC output voltage of converter 103, and ID is a current flowing in the DC circuit (FIG. 16(C)). VDI is a DC input voltage of inverter 104 (FIG. 16(D)). IU (FIG. 16(E)) is a current that is supplied to the U phase of synchronous motor 164 from the U phase of inverter 104. The same currents as current IU also flow in the V phase and W phase of inverter 104, but lagging in phase by 120.degree. and 240.degree. with respect to current IU, respectively. VUV (FIG. 16(F)) is a UV between-phase voltage of inverter 104. The VW between-phase voltage and the WU between-phase voltage of inverter 104 have the same waveform as voltage VUV, but lagging in phase by 120.degree. and 240.degree., respectively. TQ is a torque generated by synchronous motor 164 (FIG. 16(G)).
In the construction of the prior art example described above, converter 103 and inverter 104 are constituted by thyristor converters. Thyristors have the advantages that they allow large current to flow with low forward voltage in the ON condition and that they show little switching loss. Using these thyristors, high-voltage large-capacity power converters of high efficiency and small size can be manufactured cheaply. Furthermore, the stress on the devices during switching is small, leading to high reliability. That is, the externally-commutated power converter as shown in FIG. 15 has considerable advantages in comparison with self-commutated power converters, wherein the converter itself is also large, and which are of high cost with the increase of switching loss.
However, since, with the thyristors used in the circuit shown in FIG. 15, commutation is performed depending on an AC voltage as they have no self-turn-off capability, the thyristor converter consumes lagging reactive power. When a thyristor converter is employed for control of an AC motor, a synchronous motor must therefore be used which is operated in a leading-phase region, so that lagging reactive power is supplied by the synchronous motor. However, since the current waveform flowing in the synchronous motor is a square wave as shown by IU in FIG. 16(E), the torque generated by the synchronous motor has a large pulsating component as shown by TQ of FIG. 16(G). This may cause vibration and/or noise due to resonance.